1. Field of the Invention
The present invention relates to an oil-pressure controlling device for an earthmoving machine that performs a load sensing control in which the pressure differential between the discharge pressure of a hydraulic pump and the load pressure of hydraulic actuators are maintained at a set value.
2. Description of the Related Art
FIG. 2 is a perspective view of the peripheral parts of a blade provided in the front part of the chassis of a bulldozer.
The bulldozer carries out work including the digging and shifting of earth as well as the leveling of the ground surface following digging using a blade 3 (moldboard) attached to the front part of the vehicle main body.
A left and right pair of tilt cylinders 4, 5 are provided between the blade 3 and the vehicle main body.
When the two tilt cylinders 4, 5 are simultaneously extension-driven in the same direction the blade 3 is moved to either a pitch dump position (forward tilt position) or a pitch back position (backward tilt position).
In addition, when one tilt cylinder is extension-driven with the other tilt cylinder in the stationary state, the blade 3 is moved to a position in which either the right-end part or the left-end part thereof tilts downward (left tilt position or right tilt position). This is known as a single tilt operation.
In addition, when one tilt cylinder is extension-driven and the other tilt cylinder is simultaneously retraction-driven, the actuation speed of the tilt operation of the blade 3 increases. This is known as a dual tilt operation. Japanese Patent Application Laid-Open No. 2002-275931 provides a description of the elements comprising the implementation of a dual tilt operation.
A load sensing controller is installed in a hydraulic circuit for implementing the drive control of the blade 3.
The load sensing control referred to here constitutes a control for changing the capacity (cc/rev) of the hydraulic pump, or more specifically the tilt-rotated position of a swash plate thereof, in such a way that the pressure differential ΔP (=Pp−PL) between the discharge pressure Pp of the hydraulic pump and the load pressure PL of the tilt cylinders 4, 5 is maintained at a set value ΔPLS.
The load sensing controller of this hydraulic circuit constitution comprises a variable capacity hydraulic pump, a directional flow control valve for controlling the flow of the flow of hydraulic oil supplied from the hydraulic pump to the tilt cylinders, and capacity control means for controlling the discharge amount (cc/rev) discharged from the hydraulic pump per turn of the pump and, serving as the capacity control means, a drive cylinder device for driving the swash plate of the hydraulic pump and a load sensing control valve (LS valve) for controlling the drive of the drive cylinder device. Here, the load sensing control valve comprises a pair of opposing drive parts, the discharge pressure Pp of the hydraulic pump and the load pressure PL of the hydraulic actuators are led to respective drive parts, and a spring of a spring force equivalent to the constant pressure differential ΔPLS is arranged in the drive part to which the load pressure PL is led.
In a hydraulic circuit constitution such as this, when hydraulic oil is supplied from the hydraulic pump, hydraulic oil is supplied to the tilt cylinders 4, 5 by way of the directional flow control valve, the tilt cylinders 4, 5 are driven, and the blade 3 is actuated. When the blade 3 is actuated, the load sensing control valve operates in accordance with the pressure differential ΔP between the discharge pressure Pp of the hydraulic pump and the load pressure (maximum load pressure) PL of the tilt cylinders 4, 5 and the drive cylinder device is driven. As a result, the capacity of the hydraulic pump (tilt-rotated position of the swash plate) is changed in such a way as to maintain the pressure differential ΔP at the constant pressure differential ΔPLS set by the spring.
Taking the opening area of the spool of the directional flow control valve as A and the resistance coefficient thereof as c, the discharge flow Q (1/min) of the hydraulic pump is expressed by equation (1) below:Q=c·A·√(ΔP)  (1)
Because the pressure differential ΔP is maintained by the load sensing control valve as a constant (ΔPLS), the hydraulic pump discharge flow Q is changed only by change in the opening area of the directional flow control valve spool A.
When the tilt operating lever is operated from the neutral position, the opening area of the directional flow control valve spool A increases in accordance with the extent of this operation and the pump flow Q increases in accordance with the increase of the opening area A. The pump flow Q at this time is determined by the extent of the operation of the tilt operating lever only and is unaffected by the magnitude of the tilt cylinder load. By the provision of a load sensing valve in this way the pump flow Q is changed in accordance with the wishes of the operator (in accordance with the operating position of the operating level) without fluctuation due to load and, accordingly, the fine control characteristics thereof, in other words the operability in the middle operating range, is improved.
Load sensing controllers are also installed in hydraulic excavators. Controllers installed in hydraulic excavators change the magnitude of the constant pressure differential ΔPLS established by the spring of the load sensing control valve in accordance with work type (work mode).
In addition, blades of both a standard specification and a large blade specification are used in bulldozers.